GpuReadbackPlugin: Allow reading only a part of a buffer (#20133)

# Objective

- So far only full buffer reads were supported. This adds the ability to
read a part of a buffer.

## Solution

- Allow passing in a start offset and a number of bytes to read when
creating the `Readback` component.
- I also removed the unused `src_start` and `dst_start` fields from
`ReadbackSource` as they were always 0.

## Testing

- Did you test these changes? If so, how?

I extended the example to also demonstrate partial reads.

- Are there any parts that need more testing?

Can't think of any.

- How can other people (reviewers) test your changes? Is there anything
specific they need to know?

Run the `gpu_readback` example. It now also reads and prints a partially
read buffer.

- If relevant, what platforms did you test these changes on, and are
there any important ones you can't test?

Only tested on Linux.

---

## Showcase

Example output:

<details>
  <summary>Click to view showcase</summary>

```
2025-07-14T14:05:15.614876Z  INFO gpu_readback: Buffer [257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272]
2025-07-14T14:05:15.614921Z  INFO gpu_readback: Buffer range [261, 262, 263, 264, 265, 266, 267, 268]
2025-07-14T14:05:15.614937Z  INFO gpu_readback: Image [257, 258, 259, 260, 261, 262, 263, 264, 265, 266, 267, 268, 269, 270, 271, 272, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
```

</details>

crates/bevy_render/src/gpu_readback.rs

@@ -77,7 +77,10 @@ impl Plugin for GpuReadbackPlugin {
 #[derive(Component, ExtractComponent, Clone, Debug)]
 pub enum Readback {
     Texture(Handle<Image>),
-    Buffer(Handle<ShaderStorageBuffer>),
+    Buffer {
+        buffer: Handle<ShaderStorageBuffer>,
+        start_offset_and_size: Option<(u64, u64)>,
+    },
 }
 
 impl Readback {
@@ -86,9 +89,21 @@ impl Readback {
         Self::Texture(image)
     }
 
-    /// Create a readback component for a buffer using the given handle.
+    /// Create a readback component for a full buffer using the given handle.
     pub fn buffer(buffer: Handle<ShaderStorageBuffer>) -> Self {
-        Self::Buffer(buffer)
+        Self::Buffer {
+            buffer,
+            start_offset_and_size: None,
+        }
+    }
+
+    /// Create a readback component for a buffer range using the given handle, a start offset in bytes
+    /// and a number of bytes to read.
+    pub fn buffer_range(buffer: Handle<ShaderStorageBuffer>, start_offset: u64, size: u64) -> Self {
+        Self::Buffer {
+            buffer,
+            start_offset_and_size: Some((start_offset, size)),
+        }
     }
 }
 
@@ -193,9 +208,8 @@ enum ReadbackSource {
         size: Extent3d,
     },
     Buffer {
-        src_start: u64,
-        dst_start: u64,
         buffer: Buffer,
+        start_offset_and_size: Option<(u64, u64)>,
     },
 }
 
@@ -266,16 +280,30 @@ fn prepare_buffers(
                     });
                 }
             }
-            Readback::Buffer(buffer) => {
+            Readback::Buffer {
+                buffer,
+                start_offset_and_size,
+            } => {
                 if let Some(ssbo) = ssbos.get(buffer) {
-                    let size = ssbo.buffer.size();
+                    let full_size = ssbo.buffer.size();
+                    let size = start_offset_and_size
+                        .map(|(start, size)| {
+                            let end = start + size;
+                            if end > full_size {
+                                panic!(
+                                    "Tried to read past the end of the buffer (start: {start}, \
+                                    size: {size}, buffer size: {full_size})."
+                                );
+                            }
+                            size
+                        })
+                        .unwrap_or(full_size);
                     let buffer = buffer_pool.get(&render_device, size);
                     let (tx, rx) = async_channel::bounded(1);
                     readbacks.requested.push(GpuReadback {
                         entity: entity.id(),
                         src: ReadbackSource::Buffer {
-                            src_start: 0,
-                            dst_start: 0,
+                            start_offset_and_size: *start_offset_and_size,
                             buffer: ssbo.buffer.clone(),
                         },
                         buffer,
@@ -307,17 +335,11 @@ pub(crate) fn submit_readback_commands(world: &World, command_encoder: &mut Comm
                 );
             }
             ReadbackSource::Buffer {
-                src_start,
-                dst_start,
                 buffer,
+                start_offset_and_size,
             } => {
-                command_encoder.copy_buffer_to_buffer(
-                    buffer,
-                    *src_start,
-                    &readback.buffer,
-                    *dst_start,
-                    buffer.size(),
-                );
+                let (src_start, size) = start_offset_and_size.unwrap_or((0, buffer.size()));
+                command_encoder.copy_buffer_to_buffer(buffer, src_start, &readback.buffer, 0, size);
             }
         }
     }

examples/shader/gpu_readback.rs

@@ -74,7 +74,7 @@ fn setup(
     mut buffers: ResMut<Assets<ShaderStorageBuffer>>,
 ) {
     // Create a storage buffer with some data
-    let buffer = vec![0u32; BUFFER_LEN];
+    let buffer: Vec<u32> = (0..BUFFER_LEN as u32).collect();
     let mut buffer = ShaderStorageBuffer::from(buffer);
     // We need to enable the COPY_SRC usage so we can copy the buffer to the cpu
     buffer.buffer_description.usage |= BufferUsages::COPY_SRC;
@@ -110,6 +110,19 @@ fn setup(
             let data: Vec<u32> = trigger.event().to_shader_type();
             info!("Buffer {:?}", data);
         });
+
+    // It is also possible to read only a range of the buffer.
+    commands
+        .spawn(Readback::buffer_range(
+            buffer.clone(),
+            4 * u32::SHADER_SIZE.get(), // skip the first four elements
+            8 * u32::SHADER_SIZE.get(), // read eight elements
+        ))
+        .observe(|trigger: On<ReadbackComplete>| {
+            let data: Vec<u32> = trigger.event().to_shader_type();
+            info!("Buffer range {:?}", data);
+        });
+
     // This is just a simple way to pass the buffer handle to the render app for our compute node
     commands.insert_resource(ReadbackBuffer(buffer));